A high-carbohydrate diet lowers the rearing cost and decreases the ammonia emission into the environment, whereas it can induce liver injury, which can reduce harvest yields and generate economic losses in reared fish species. Macroalgae Saccharina japonica (SJ) has been reported to improve anti-diabetic, but the protective mechanism of dietary SJ against liver injury in fish fed a high-carbohydrate diet has not been studied. Therefore, a 56-day nutritional trial was designed for swamp eel Monopterus albus, which was fed with the normal diet [20% carbohydrate, normal carbohydrate (NC)], a high carbohydrate diet (32% carbohydrate, HC), and a HC diet supplemented with 2.5% SJ (HC-S). The HC diet promoted growth and lowered feed coefficient (FC), whereas it increased hepatosomatic index (HSI) when compared with the NC diet in this study. However, SJ supplementation increased iodine contents in muscle, reduced HSI, and improved liver injury, such as the decrease of glucose (GLU), total bile acid (TBA), and alanine aminotransferase (ALT) in serum, and glycogen and TBA in the liver. Consistently, histological analysis showed that SJ reduced the area of lipid droplet, glycogen, and collagen fiber in the liver (p < 0.05). Thoroughly, the underlying protective mechanisms of SJ supplementation against HC-induced liver injury were studied by liver transcriptome sequencing coupled with pathway analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the differentially expressed genes (DEGs), such as the acetyl-coenzyme A synthetase (acss1), alcohol dehydrogenase (adh), interferon-induced protein with tetratricopeptide repeats 1 (ifit1), aldo-keto reductase family 1 member D1 (akr1d1), cholesterol 7-alpha-monooxygenase (cyp7a1), and UDP-glucuronosyltransferase (ugt), indicated that the pathway of glycolysis/gluconeogenesis was the main metabolic pathway altered in the HC group compared with the NC group. Meanwhile, hepatitis C, primary BA biosynthesis, and drug metabolism-cytochrome P450 were the three main metabolic pathways altered by SJ supplementation when compared with the HC group. Moreover, the BA-targeted metabolomic analysis of the serum BA found that SJ supplementation decreased the contents of taurohyocholic acid (THCA), taurochenodeoxycholic acid (TCDCA), taurolithocholic acid (TLCA), nordeoxycholic acid (NorDCA), and increased the contents of ursocholic acid (UCA), allocholic acid (ACA), and chenodeoxycholic acid (CDCA). In particular, the higher contents of UCA, ACA, and CDCA regulated by SJ were associated with lower liver injury. Overall, these results indicate that the 2.5% supplementation of SJ can be recommended as a functional feed additive for the alleviation of liver injury in swamp eel-fed high-carbohydrate diets.

The aim of this study was to examine the effects of dietary protein and lipid levels on the growth performance and homeostasis of the intestinal flora in Paramisgurnus dabryanus. An 8-wk 3 × 3 two-factorial experiment was conducted to investigate the interaction between dietary crude protein (CP: 30%, 35%, 40%) and ether extract (EE: 6%, 10%, 14%) on the growth rate and the intestinal microflora of P. dabryanus. A total of 2,160 fish (5.19 ± 0.01 g) were randomly allotted to 36 aquariums each with 60 fish. Fish were fed the experimental diet twice daily. Results revealed that weight gain rate (WGR), specific growth rate (SGR), protein efficiency ratio and net protein utilization significantly increased when increasing protein levels from 30% to 40% (P < 0.05). Both WGR and SGR enhanced first but reduced thereafter with maximum value at 10% lipid level as dietary lipid increased from 6% to 14% (P < 0.05). Significant interactions between protein and lipid were found with feed conversion rate, lipid efficiency ratio and net lipid utilization (P < 0.05). At the phylum level, Proteobacteria and Actinobacteria were the dominant bacteria; at the genus level, Burkholderia-Caballeronia-Paraburkholderia was the dominant bacteria. Fish fed the diet containing 10% lipid had a higher abundance of Proteobacteria and unclassified_f_Eenterobacteriaceae than those fed the 14% lipid diet, and a higher abundance of Rhodobacter than those fed the 6% lipid diet (P < 0.05). Analysis of the predicted functions showed that metabolism in the intestine of fish in the CP40EE10 group was more active than that in CP30EE14 group. Polynomial regression analysis found that a diet containing 40.87% protein and 9.88% lipid can be considered optimal for P. dabryanus.

This study aimed to analyze the intestinal flora of male versus female swamp eels, which have the unique characteristic of sex reversal. Same-aged swamp eels from the same parents, kept under the same conditions in terms of environment, diet, etc., were used as the study materials. After breeding for 1 year, 18 healthy swamp eels (nine males and nine females), weighing 39.4 ± 2.3 g, were selected. The intestinal contents of three swamp eels of the same sex were combined and labeled as follows: FM1-FM3 (n = 3) for females and MM1-MM3 (n = 3) for males. High-throughput sequencing was used to research the v3-v4 region of 16S rDNA in the intestinal flora. The results suggested significant differences in the structure, composition, and diversity of the intestinal flora of male versus female swamp eels. The relative abundances of Actinobacteria and Spirochaetes in female swamp eels were significantly higher (p < 0.05) than in male swamp eels at the phylum level. The relative abundances of Mycobacterium, Bacillus, and Vagococcus in female swamp eels were considerably higher (p < 0.05) than in male swamp eels at the genus level. The Alpha diversity of females was higher than that of males, and the Shannon index of females was also significantly higher (p < 0.05, Tukey’s HSD) than that in males. Investigations of Beta diversity, including NMDS ordination, UPGMA cluster analysis, and PLS-DA analysis, showed that female and male swamp eels could be clearly distinguished based on significant differences in intestinal flora between the FM group and the MM group. LEfSe analysis showed that the dominant bacteria were Cetobacterium in male swamp eels and Clostridium_sp_ND2, Mycobacterium, Alphaproteobacteria, and Bacilli in female swamp eels. The results showed dramatic differences in the intestinal flora between the sexes, which indicates the need for a more in-depth study on sex reversal in the future.

The subject of this study was to explore the optimum requirements of loach (Paramisgurnus dabryanus) regarding dietary proteins and lipids and discuss the underlying mechanism. We designed nine diets to determine the effects of different levels of dietary crude protein (CP: 30%, 35%, and 40%) and ether extract (EE: 6%, 10%, and 14%) on the growth performance and metabolism of P. dabryanus. In total, 2160 healthy P. dabryanus (5.19 ± 0.01 g) were divided into nine groups with four replications at 60 fish per barrel stocking density. The trial lasted for eight weeks. Serum and liver samples were gathered for metabolomic and transcriptomic analyses. The results showed that the specific growth rate of P. dabryanus in the CP40EE10 group was the fastest and notably higher than that in other groups (P&lt; 0.05). Analysis of the metabolome results found that the mTOR signaling pathway, glycerophospholipid metabolism, D-arginine and D-ornithine metabolism were significantly enriched pathways in the CP40EE10 group compared with the other groups (P&lt; 0.05). Moreover, the transcriptomic analysis of differentially expressed genes (DEGs) showed that the expression of ARG (arginase) involved in protein synthesis was significantly upregulated in the CP40EE10 group compared to the slowest growing group (P< 0.05). Additionally, the expression of SPLA2 (secretory phospholipase A2) involved in lipid metabolism and FBP (fructose-1,6-bisphosphatase) involved in glucose metabolism were all significantly downregulated in the CP30EE6 group compared with the CP40EE10 group (P< 0.05). Furthermore, the analysis of differentially expressed metabolites (DEMs) and DEGs co-enriched in the KEGG pathway revealed that the significantly enriched pathways were arginine and proline metabolism, glycerophospholipid metabolism, and glycolysis/gluconeogenesis in CP40EE10 compared with other groups (P< 0.05). We conclude that including 40% CP and 10% EE in the P. dabryanus diet could result in a better growth rate. We hypothesized from metabolomic and transcriptomic analyses that the CP40EE10 diet might promote the growth of P. dabryanus by promoting protein synthesis, lipid metabolism, and energy production.

To assess the dietary arginine requirement of Monopterus albus (21 ± 0.03 g), we compared six diets containing different arginine levels (2.03%, 2.58%, 3.16%, 3.63%, 4.24%, and 4.88%). The results revealed that dietary arginine content had a significant effect on weight gain (WG), specific growth rate (SGR), IL-1β, IL-10, lysozyme, and complement C3 activity in Monopterus albus ( ). The WG and SGR increased with increasing dietary arginine levels, and the regression analysis found that the optimum dietary arginine level for growth of rice field eel was 4.28–4.35% of the dry diet. However, the C3 and LZM activities peaked in the 2.58% arginine diet group ( ). The dietary arginine level had no significant effect on the viscerosomatic index, hepatosomatic index, condition factor, survival rate, nitric oxide, total nitric oxide synthase, insulin, growth hormone, trypsin, lipase, or amylase activities ( ). Arginine can significantly promote Monopterus albus growth; however, the immunity of Monopterus albus decreases as the arginine level exceeds 2.58%. A comprehensive assessment of growth and immunity recommends the optimum arginine level for Monopterus albus was 3.16–3.63% of the dry diet, corresponding to 6.58–7.70% of the dietary protein.